currently, I have a router with Tomato loaded. I also have two 48 port switches behind a port 1 and port 2.

This is a the rental properties to share the internet access with fiber 100M down and 50M up. I know if I can assign each device with 255.255.255.255 as network mask, then they won't be able to see each other. However, I am using 255.255.252.0 in the DHCP which means people can go to their "my network place" and see other computers or devices. Is there any way, I can disable the broadcasting feature so each device won't be able to see others in the router? even 255.255.252.0 is used? What should I do to make it happens? thanks!

The best approach would be to give each client their own Tomato router and do double NAT. This way you can assign them each a capped package of 10X1.5 and also qos traffic shape them and control their connection counts. This will of course firewall them from each other while it is at it.